Preheat oven to 180°C and line two 20cm tins with parchment.
Cream butter, sugar, and passion fruit in a bowl using a whisk.
Sift rice flour, xanthan gum, and baking powder together in another bowl.
Beat in eggs and flour mixture gradually before folding in desiccated coconut.
Divide batter into tins and bake for 40-45 minutes until golden.
Whisk butter, icing sugar, and lime zest until fluffy to make buttercream.
Beat in coconut cream and rum if using until combined.
Dissolve sugar in passion fruit and lime juices to create a syrup.
Prick warm cakes with a skewer and pour over the strained syrup.
Sandwich cooled cake layers with coconut buttercream.
Scatter reserved seeds on top and slice to serve.